Dilemma of Non‐Steady State in Lakes – Development and Predictability of In‐Lake P Concentration in Dimictic Lake Scharmützelsee (Germany) after Abrupt Load Reduction

Based on measured and calculated long‐term data on external phosphorus (P) load (1920–2009), hypolimnetic P accumulation and trophic parameters for the dimictic Lake Scharmützelsee, we aimed to identify factors which cause variable P net sedimentation and the importance of internal P loading for different time periods especially focusing on non‐steady state after abrupt external load reduction in 1988. P retention (R) decreased from 0.85 during the high external P loading (0.37 g m^–2 a^–1) phase (1950–1988) to 0.71 during the following transient phase, and increased to 0.81 for the present recovery phase (0.17 g m^–2 a^–1) beginning in 2003. Mean net sedimentation coefficients for the same periods were 0.47, 0.22 and 0.30. Our results show that a) empirical models overestimate R during the high loading phase and underestimate R during the transient phase after load reduction, and b) the application of simple one‐box models which assume that a portion of in‐lake P stock is retained requires the consideration of the variability of the net sedimentation coefficient. We identified multiple reasons for variable gross sedimentation (e.g., particle trapping in the elongated lake; efficient accumulation of sewage P) as well as release of P (delayed release of mobile P from sediment; changes in plant colonization and food webs), so that their relation (net sedimentation) varies. Despite a new equilibrium reached in 2003, it is still unclear when the lake will reach mesotrophic reference conditions and a good ecological status. Historical data and elevated Cl^– concentration (22 mg L^–1) indicate that P import from sewage contaminated groundwater still continues, so that the mean in‐lake P concentration is still too high (53 µg L^–1), and biological structures have not fully recovered yet. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)